Pistons which are used as movable components in combustion chambers of internal combustion engines are subject to high mechanical and thermal loads. This makes it virtually imperatively necessary for said pistons to be produced without so-called defects which could considerably reduce both wear resistance and the service life of the piston. This applies in particular to defects which arise in the region of that piston region, known to a person skilled in the art as “piston depression”, which axially delimits the combustion chamber of the internal combustion engine in the movement direction of the piston.
Here, in the present case, the expression “defects” is to be understood to mean any type of cracks, pores, fractures etc. in the material of the piston. Since such defects normally arise intrinsically, that is to say below the piston surface, within the piston, nondestructive detection—that is to say the detection of any defects present in the piston—is normally associated with cumbersome detection processes. Such conventional nondestructive testing methods are normally based on the use of eddy current sensors. Such sensors are based on the detection of an electromagnetic field generated with the aid of a coil which, for this purpose, has an electrical alternating current pass through it. Through interaction of the electromagnetic field with the material of the piston in the region of the piston surface, it is possible for defects that are not visible from the outside to be detected by virtue of the electromagnetic field being analyzed after its interaction with the piston material.
In the case of such conventional testing methods, it has proven to be disadvantageous that, typically, considerable disturbance influences are encountered, for example the lift-off effect between sensor and piston surface, complex aperture characteristics of the sensors, and only a small depth of penetration of the electromagnetic field into the piston surface, which impedes the detection specifically of defects with small dimensions. Furthermore, only the detection of open defects or defects in the immediate vicinity below the piston surface is ensured. By contrast, defects that have formed even a few tenths of a millimetre below the surface of the piston normally remain hidden to eddy current sensors. The grain structure of the piston microstructure often leads, in conjunction with the differentiating action of the differential probes used, to increased microstructure noise in the eddy current signal, which has an adverse effect on the defect detection sensitivity.